An electrical transformer transfers power from a primary winding to a secondary winding through the interaction of the windings' magnetic fields, with no conductive electrical contact. For many years, the notion of transferring power magnetically from a primary winding to a physically separable secondary winding has intrigued inventors. Reasons for coupling power magnetically include the safety and convenience of avoiding open electrical contacts and degradation that can afflict those contacts.
Such technology has been utilized to recharge batteries in portable devices. For example, one application is in connection with recharging an electric toothbrush, as described, for example, in U.S. Pat. No. 3,840,795 issued to Roszyk et al. During charging, the primary coil in the toothbrush's charging stand is in a close and predetermined physical relationship with the secondary coil in the handheld unit to be recharged. It has been found that using close and predetermined positioning can provide a tight, transformer-like magnetic coupling. This concept has also been applied in connection with recharging wireless telephones, as described, for example, in U.S. Pat. No. 5,396,538 issued to Hong.
The relatively tight magnetic coupling of primary and secondary windings in known arrangements is provided by the precise physical relationship between the charging unit and the receiving unit. However, difficulty arises when attempting to provide a magnetic power source device that can couple with a variety of receiving devices without utilizing a close and predetermined physical relationship. In other words, when the precise physical relationship between the primary and the secondary is not known, tight magnetic coupling may not be achieved. For example, much of the magnetic flux produced by the primary winding may not be coupled to the secondary winding, thereby reducing the efficiency of power transfer and making the task of creating a sufficiently strong magnetic field in the vicinity of the secondary winding more difficult. Also, the strength of the field reaching the secondary winding can vary considerably with changes in the secondary winding's position relative to the primary winding.
There have been attempts to provide uniform magnetic fields in order to improve coupling where the physical relationship between a primary and secondary winding is not specifically predetermined. See, for example, U.S. Pat. No. 6,906,495 issued to Cheng et al. and U.S. Pat. No. 7,211,986 issued to Flowerdew and Huddart. However, in such schemes, a decreasing portion of the generated magnetic field couples to the secondary winding as the physical configuration becomes more general (less specific) and more removed from the condition of a close primary-secondary coupling. In other words, it is desirable to improve coupling where the physical relationship between a primary and secondary winding is not specifically predetermined.
Relatively poor coupling can be represented in a transformer model by reduced primary to secondary mutual inductance and a corresponding increase in the series leakage inductances. In many such cases of imperfect coupling, the amount of magnetic flux that does not link the windings (and therefore contributes to the leakage inductances) can be significant in comparison to the flux that does link the windings (and therefore can transfer power). The voltage drops from the series impedances of the primary and secondary leakage inductances and the associated reactive driving power can be reduced by resonating the primary or secondary winding, or both, with associated capacitors.
There have been attempts to improve coupling efficiency by maintaining at least an approximate match between the drive frequency associated with a primary circuit and the resonance of the primary and/or secondary circuits. See, for example, U.S. Pat. Nos. 6,028,413 issued to Brockmann, 6,825,620 issued to Keunnen et al., 6,906,495 issued to Cheng et al., 7,211,986 issued to Flowerdew et al. 6,972,543 issued to Wells; and “A Contactless Electrical Energy Transmission System”, IEEE Transactions on Industrial Electronics, vol. 46, pp. 23-30, February 1999 by Pedder et al. However, known approaches may be complex and/or may not provide desired results. In other words, it is desirable to provide improved systems and methods that can maintain at least an approximate match between the drive frequency associated with a primary circuit and the resonance of the primary and/or secondary circuits.
There have been attempts to provide inductively rechargeable batteries that include a secondary winding for inductively receiving charging power from a primary winding. See, for example, U.S. Pat. Nos. 6,208,115 issued to Binder, 6,498,455 issued to Zink et al., 6,906,495 issued to Cheng et al. However, it has been discovered that known systems and methods do not address power losses associated with shunting of the magnetic path by the storage cell materials or reducing potential losses from the flux-concentrating magnetic material itself. Providing an inductively rechargeable battery that addresses such issues is desirable. Further, providing an inductively rechargeable battery with improved volumetric efficiency is also desirable.
There have been attempts to provide control over the charging process by allowing a battery to communicate its charging needs to primary side circuitry. See, for example, U.S. Pat. Nos. 5,396,538 issued to Hong, 5,952,814 issued to Van LerBerghe, 6,118,249 issued to Brockmann et al. However, known systems and methods do not provide for battery charge need sensing that is simply implemented, does not require additional data paths, and does not significantly interfere with the charging operation. Providing an inductively rechargeable battery system that addresses such issues is desirable.